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Modelling the sources of organic material, processes and timescales leading to arsenic contamination of circum-Himalayan groundwaters
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
Arsenic contamination of circum-Himalayan groundwater is leading to one of the greatest humanitarian disasters of modern times, poisoning at least 70 million people who are mostly poor and rural. The groundwater is hosted in Holocene aquifers consisting of Himalayan sediments deposited by the great Asian rivers in deltaic environments. Arsenic is released when organic material (OM) reacts with the iron-oxide minerals co-deposited in the sediments onto which arsenic is adsorbed. The source of OM is one of the most important questions facing researchers and policy makers. There are generally accepted to be three potential sources of OM: 1) sedimentary bound OM (SOM) co-deposited with sediments; 2) thermally mature petroleum upwelled from reservoirs below the aquifers; 3) dissolved organic carbon (DOC) some of which might be drawn in to the aquifer through modern pumping and irrigation. In this thesis the nature of organic material in the aquifer is researched and the processes and timescales which lead to arsenic release are studied. Here evidence for a new conceptual model of arsenic release is presented. Isotopic tracing combined with a new geochemical model and organic geochemical techniques, shows that OM driving arsenic release pre-dates agriculture in the region and was from natural grasslands in the early Holocene. The geochemical model utilises strontium isotopes to correct the radiocarbon age of dissolved inorganic carbon (DIC) to find only the age and isotopic signature of DIC from oxidation of organic material. This shows that DIC from oxidation of OM was from the early Holocene and had an isotopic signature consistent with the early Holocene SOM in this region. A study of the sediments in the region built upon a geomorphological history shows that the most oxidised SOM is from early Holocene sediments. Thus both techniques separately indicate that pre-agricultural organic material drove arsenic release. This conceptual model however reveals the “arsenic sand paradox”, because whilst release is from early Holocene clays, today highest concentrations of arsenic are in younger sands. Explaining this paradox is the most important next step leading on from this research.
Layman's Abstract
In South and South East Asia, from Pakistan at the western periphery, to Cambodia and southern China at the eastern, groundwater, which is commonly used for cooking and drinking, is contaminated with arsenic. This has led to one of the largest natural health catastrophes of modern times. Arsenic poses a huge risk to human health; where the risks associated with this groundwater are similar to the risks from smoking. Researchers have been interested in the causes of these high concentrations of arsenic for about two decades, and it is now widely accepted that the cause of arsenic release is iron oxide minerals onto which arsenic is attached (adsorbed) dissolving into groundwater due to reactions between the minerals and organic material. A major concern for researchers has been that human action might increase arsenic release into these groundwaters; researchers are concerned that pumping water for farming or drinking might draw organic material into the aquifer from the surface and thus increase the number of reactions and amount of iron oxides dissolving, leading to increased levels of arsenic in the groundwater. The results from this research clearly demonstrate that this is not occurring, and that the reactions between iron oxide minerals and organic material occurred before 6,000 years ago, which pre-dates any agricultural activity in the area. Therefore this research has shown that human activity probably does not increase the risk of arsenic contamination to these groundwaters.